The invention relates to an electronic circuit comprising a differential pair which is provided with degeneration means for degenerating a transconductance of the differential pair, and an auxiliary circuit for providing a control voltage to a control electrode of the degeneration means.
Such an electronic circuit is known from U.S. Pat. No. 5,642,078. The known circuit is shown in FIG. 1 and comprises inter alia a differential pair consisting of a field effect transistor 22 and a field effect transistor 23. The differential pair further comprises a field effect transistor 16 which is connected by a source to the source of the field effect transistor 23 and which is connected by a drain to the source of the field effect transistor 22. The gate of the field effect transistor 16 is connected to a bias control circuit. The required direct currents for the differential pair are supplied by a current source 20 and a current source 21. The field effect transistor 16 is set for its linear operation range and serves to degenerate the transconductance of the differential pair.
It is a disadvantage of the known electronic circuit that the transconductance of the differential pair is not accurately defined.
It is an object of the invention to provide an electronic circuit comprising a differential pair whose transconductance is accurately defined.
According to the invention, the electronic circuit mentioned in the opening paragraph is for this purpose characterized in that the auxiliary circuit is designed for providing a DC biasing of the differential pair, and in that the auxiliary circuit is designed such that the transconductance of the differential pair is substantially independent of the value of the control voltage and of the DC biasing of the differential pair.
Since not only the control voltage at the control electrode of the degeneration means is regulated, as in the known circuit, but also the DC currents of the differential pair are regulated, it can be achieved that the transconductance of the differential pair becomes equal to a desired reference value. The reference value may be obtained, for example, through the use of a reference resistance. The reference resistance may be obtained in various alternative ways, for example through the use of a ratio of resistance values, a capacitor, a ratio of capacitance values, a voltage source, a ratio of voltage values, a current source, a ratio of current values, etc.
An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit comprises a further differential pair comprising further degeneration means for degenerating a transconductance of said further differential pair, and in that the auxiliary circuit is furthermore designed for providing a further control voltage to a control electrode of the further degeneration means, and in that the control voltage is dependent on said further control voltage.
An electronic circuit according to the invention is implemented thereby in a simple manner. If so desired, the degeneration means and the further degeneration means may be constructed in a similar manner. It is also possible to choose the control voltage to be equal to the further control voltage.
An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit comprises current bias means for providing a desired current ratio between a first current branch and a second current branch of the further differential pair.
This renders it simpler to obtain a correct DC biasing of the differential pair.
An embodiment of an electronic circuit according to the invention is characterized in that the current bias means are provided with a first input which is coupled to the first current branch, a second input which is coupled to the second current branch, and an output which is coupled to the control electrode of the further degeneration means.
The current bias means control the control electrode of the further degeneration means such that the ratio of the respective currents of the first and the second current branch is equal to the desired current ratio.
An embodiment of an electronic circuit according to the invention is characterized in that the auxiliary circuit furthermore comprises voltage bias means for providing a first bias voltage to a first input of the further differential pair and a second bias voltage to a second input of the further differential pair, and in that the further differential pair further comprises first current supply means connected in series with the first current branch and second current supply means connected in series with the second current branch, and in that the currents supplied by said first current supply means and second current supply means are dependent on the difference between the first bias voltage and the second bias voltage.
It is achieved thereby that not only the desired current ratio of the first and the second current branch is equal to the desired current ratio, but that also the absolute values of the currents through the first and the second current branch are defined.
An embodiment of an electronic circuit according to the invention is characterized in that said currents are approximately linearly dependent on the difference between the first bias voltage and the second bias voltage.
This renders it possible to generate said currents in a simple manner.
An embodiment of an electronic circuit according to the invention is characterized in that the degeneration means comprise a field effect transistor which is set for its linear operation range, and in that the further degeneration means comprise a further field effect transistor which is set for its linear operation range.
A further linearizing of the differential pair is realized thereby, so that the distortion of the differential pair is reduced.
An embodiment of an electronic circuit according to the invention is characterized in that the value of the first bias voltage corresponds to the highest value of the first bias voltage at which the electronic circuit can still function correctly, and in that the value of the second bias voltage corresponds to the lowest value of the second bias voltage at which the electronic circuit can still function correctly.
This has the advantage that the differential pair continues to function correctly also at the widest possible signal control range at the inputs of the differential pair.
A further advantageous embodiment of an electronic circuit according to the invention is defined in claim 10.